The research encompassed within this program project is organized around our hypothesis that physiol9ogical gene therapy for sickle cell disease may be achieved by genetically reversing the switch from fetal (HbF) to adult (HbS) hemoglobin through gene transfer into hematopoietic stem cells. During the next five years of research, our goal is to significantly advance the de4velopment of gene therapy by achieving a growing understanding of the molecular mechanisms that control red cell formation, the regulatory factors that determine the proportion of gamma and beta-globin expression and the biological controls that modulate stem cell behaviors and transduction. "Signaling by the Epo Receptor in Erythropoiesis, the proposed research seeks to understand the pathways that emanate from the erythropoietin receptor by defining signaling components distal to Jak-2, which interacts with the membrane proximal portion of the receptor, and to define both redundant and non-redundant functions of the membrane distal part of the receptor. "Hematopoietic RING Finger 1 (HERF1) in Erythropoiesis", the research is focused around defining the role of HERF1 a novel, erythroid specific RING protein, thereby gaining insights into the molecular pathways that control the terminal stages of erythroblast maturation. "Identification and Characterization of Factors which Modulate gamma-Globin Gene Expression", experiments are focused on characterizing proteins that have been implicated in switching mechanisms and determining whether such proteins can be used to modulate the relative balance of gamma and beta synthesis in maturing erythroblasts. "In Vivo Selection of Transduced Hematopoietic Stem Cells", a selection system based on variants of dihydrofolate reductase, has been developed that allows amplification of genetically modified hematopoietic cells. The mechanism of such amplification will now be investigated in the murine model and the DHFR selection system will also be adapted for use in a non-human primate model in the context of ultimately attempting to advance this approach to clinical use. "Gene Transfer into Hematopoietic Stem Cells," the proposed experiments are focused on evaluation of standard (MuLV) retroviral vectors to lentiviral vectors with respect to their relative ability to transfer genes into primitive repopulating cells from patients with sickle cell disease and to express a therapeutic gene in maturing erythroblasts. Research in animal models of human hemoglobin disorders will test the hypothesis that retroviral mediated gene transfer and amplification of a population of genetically modified cells can be used to effectively correct the disease phenotype. The research is supported by an Administrative Core and three scientific Core's that provide purified stem cells, standardized vector preparations or access to unique animal models. Through this coordinated program of research we anticipate substantial progress toward the ultimate goal of successful gene therapy for sickle cell disease.